Diffusion of fluorides in human dental enamel in vitro

The diffusion of fluoride (F) from sodium fluoride, sodium fluorosilicate, and sodium monoflurophosphate solutions (containing 0.1 per cent F in physiological saline) was studied using a two-chamber diffusion cell separated by enamel membrane. The diffusion coefficient, D in cm2s-1, of F was determined under steady-state conditions over 3 weeks. The D value of F for NaF was significantly higher than for Na2SiF6 and Na2PO3F (p less than 0.001; unpaired t-test). F for acidic Na2SiF6 diffused more rapidly than for Na2PO3F (p less than or equal to 0.001), presumably as HF molecule. Despite the F in the tested solutions being in different forms, i.e. simple ion in NaF or mainly complex ions as in Na2SiF6 and Na2PO3F, the diffusivity of the F in enamel for these compounds was of the same order of magnitude (D = 10(-9)cm2s-1). The findings support the concept that enamel can behave as an ion-selective membrane with certain molecular-sieve effects.